System for preventing damage to a battery charger due to application of a battery with wrong polarity

In an automotive electrical system including

• an alternator having an exciter winding, exciter diodes connected to said exciter winding, a positive exciter terminal (D+), a positive rectified alternator output terminal (B+), and a negative rectified alternator terminal (B-), connected to a reference potential,
• a battery (B) having a negative terminal (A-) connected to said reference potential and a positive terminal (A+);
• a starter motor (M);
• and starter switch means (S2, C2, K2) connected to said battery so that said motor is started upon activation of said starter switch means independent of the polarity of said battery,
• apparatus for protecting said system from application of a voltage having a polarity opposite the polarity of said battery comprising
• a control switch S1;
• a protective diode (D3) conductive only when the polarity of the DC voltage is of the predetermined polarity of said battery,
• switch means including a relay (C1, K1) comprising a relay coil (C) connected in series with said protective diode and the control switch (S1) and a pair of relay contacts (K1) operative on the control of said relay coil and connected in series with the terminals (A+, A-) of said battery;
• the relay coil being connected to said system and to said blocking diode for connection of DC voltage to, and disconnection of DC from said system when said blocking diode is, respectively, conductive or non-conductive with respect to said polarity of the battery;
• and further comprising an auxiliary relay (C7, K7) having a coil connected between said positive exciter terminal (D+) and said reference potential (B-, A-),
• and a pair of auxiliary relay contacts (K7) connected in series with the relay coil (C1) of said first relay (C1, K1).